1. Field of the Invention
The present invention generally relates to an indium oxide powder and a method for producing the same. More specifically, the invention relates to an indium oxide powder used as the raw material of a high density ITO (indium oxide containing tin oxide (indium-tin-oxide)) for forming a transparent conductive film and as the raw material of IZO (indium oxide containing zinc oxide (indium-zinc-oxide)) for producing a sputtering target.
2. Description of the Prior Art
As a conventional method for producing indium oxide, there is known a method for producing indium oxide by directly oxidizing indium metal or an indium salt to decompose it. There is also known a method for producing indium oxide, the method comprising the steps of: adding a basic precipitant, such as NaOH, NH4OH or NH4HCO3, to an aqueous indium salt solution to obtain a hydroxide; drying the hydroxide; and burning the dried hydroxide in the atmosphere (Japanese Patent Laid-Open No. 2003-277052, and Japanese Patent Publication No. 7-42109).
However, in the method for directly oxidizing indium metal or an indium salt to decompose it, there is an environmental issue in that a large amount of nitrogen oxides are produced if an indium salt is decomposed, and it is difficult to sufficiently oxide the central portion of the lump of indium metal if indium metal is oxidized. In addition, indium oxide obtained by this method is a hard lump, so that it is difficult to grind the hard lump. Moreover, indium oxide obtained by the above described methods contains many rough particles, so that it is difficult to produce uniform particles.
Indium oxide powders obtained by the above described conventional methods have large particle diameters, and are hard particles. In order to obtain a high density ITO target, it is required to uniformly disperse fine indium oxide powder and fine tin oxide powder. If rough particles exist in indium oxide, the packaging property and uniformity thereof are deteriorated, so that holes and cracks are easily formed during sintering. In addition, if the presence of tin is biased, the composition of target is shifted to cause the function of tin as a carrier to be insufficient, so that conductivity is deteriorated. Therefore, indium oxide powders obtained by the above described conventional methods are bulky and hard powders, so that it is required to grind the powders by means of a beads mill or the like in order to produce a high density target. However, it takes a lot of time to carry out such a process (a wet mixing/dispersing process), so that productivity is deteriorated. In addition, there is a problem in that impurities are mixed in the powders by contamination from media, so that electric characteristics of films produced from the powders are deteriorated. Moreover, there is a problem in that uneven sintering is caused during sintering, so that cracks are easily produced.